Drives require precise positioning response and contour-forming response. To operate a drive, an operator first writes a part program describing parts, speeds, etc. Next, during a preparation step, the part program is compiled. Then, during an interpolation step, the compiled program is interpolated to generate time critical contour, milling and turning points called desired position data. Typically, the desired position data are fed to a cascaded regulator structure having control loops for current regulation, speed regulation and position regulation. A cascaded regulator structure receives the desired position data and passes the desired position data through each of the control loops to control the position of the drive.
It is preferable in a cascaded regulator structure to control the position of the drive with the lower-level control loops (e.g., the current regulator and speed regulator) and not with the position regulator since the response time of the position regulator (e.g., about 10 to 60 milliseconds) is much longer than the response times of the speed regulator (e.g., about 1 to 2 milliseconds) and current regulator (e.g., about 0.2 milliseconds). The position regulator is preferably used for the disturbance response of the system, not the position response.
Therefore, it is known to provide a feed forward control path to feed the desired position data past the position regulator to the speed regulator. Likewise, a feed forward control path may be provided to feed the desired position data past the position regulator to the current regulator. Feeding forward the position data reduces the reaction time of the system (i.e., the time between when a desired position is received and when the drive reaches the desired position). As a result, a narrower contour can be achieved with the system. However, feeding forward also causes overshoot of the desired position, which is unacceptable. Overshoot occurs since the desired position data is provided to the position regulator as well as to the lower-level regulators, resulting in a combined response at the drive.
To reduce overshoot, a modeling filter has been employed between the desired position data and the position regulator. The modeling filter attempts to cleanly model the delay between the actual position of the drive and the desired position of the drive so that, ideally, no difference between delayed desired position and actual (i.e., measured) position is fed to the position regulator, such that the position regulator does not cause the overshoot. The delay which must be modeled is caused by the feed forward paths, the lower-level control loops and the mechanical components of the drive.
A low pass filter has been used as the modeling filter, whose time constant is set to the dominant equivalent time constant of the rest of the control loop (i.e., the time constant of the speed regulator in this example), in an attempt to model the delay. However, the effectiveness of the low pass filter is dependent upon the speed of the drive. For example, for a given time constant, low drive speeds may result in a slight undershoot (for example, at the corner of a workpiece) while higher drive speeds may result in a slight overshoot. Thus, one option is to adjust the time constant to prevent overshoots. However, this option does not cleanly model the delay (i.e., the position regulator is still contributing to the position response) and, therefore, position response is slower than desirable. Another option is to set the time constant for a slight overshoot and reduce the magnitude of the overshoots by using additional desired position data. However, this option is too slow and requires more computation during the interpolation step.
Yet another option is to use a speed regulator with a reference model instead of a proportional integral response. The reference model uses an additional filter in front of the integrator of the proportional-integral speed regulator. This option makes the speed regulator faster and eases feedforward adjustment, but tuning the speed regulator is difficult and requires more knowledge of the mechanical dynamics of the system. Still another option is to use higher order filters (e.g., including a second or third order time constant). However, the higher the order of the filter used, the greater the computation time and complexity.
Therefore, what is needed is a system and method for controlling an object with improved accuracy. Further what is needed is an improved system and method for modeling the actual position of a controlled object. Such an improved system and method would allow for improved accuracy in position response on contours. Also, the improved system and method would avoid the need to accept a compromise of undershoots and overshoots, as with the low pass filter described above. Further still, the improved system and method would be more easily adjustable than prior systems.